This invention relates to a method of manufacturing an electromagnetic flow sensor and to flow sensors which can be manufactured by such a method.
As is well known, electromagnetic flow sensors can measure the volumetric flow rate of an electrically conductive fluid flowing through a measuring tube of the flow sensor. A magnetic-circuit arrangement coupled to excitation electronics produces a magnetic field of maximum density which passes through the fluid within a measurement volume in sections, particularly in the area with high flow velocity, at right angles to the direction of fluid flow, and which closes essentially outside the fluid. The measuring tube is therefore made of nonferromagnetic material, so that the magnetic field will not be adversely affected during measurements.
Due to the movement of the charge carriers of the fluid in the magnetic field, according to the magnetohydrodynamic principle an electric field of a given strength is produced at right angles to the magnetic field and to the direction of fluid flow. By two electrodes spaced in the direction of the electric field and by evaluation electronics connected to these electrodes, a voltage induced in the fluid can thus be measured. This voltage is a measure of the volumetric flow rate. To pick off the induced voltage, use is made of either galvanic electrodes which are in contact with the fluid, or capacitive electrodes, which do not contact the fluid.
The flow sensor is so designed that the induced electric field closes outside the fluid practically only via the evaluation electronics connected to the electrodes. To guide and effectively couple the magnetic field into the measurement volume, the magnetic-circuit arrangement commonly comprises two coil cores which are disposed at a distance from each other, particularly diametrically opposite each other, along a circumference of the measuring tube, and have respective free end faces located opposite each other, particularly mirror-symmetrically with respect to each other.
By means of a coil assembly connected to the excitation electronics, the magnetic field is coupled into the coil cores in such a way as to pass through the fluid flowing between the two end faces, at least in sections, at right angles to the direction of flow.
Because of the high mechanical stability required for such measuring tubes, the latter preferably consist of an external support tube of a predeterminable strength and width, particularly of a metallic support tube, whose inner surface is covered with an insulating material of predeterminable thickness, the so-called liner.
U.S. Pat. No. 3,213,685 discloses an electromagnetic flow sensor comprising:
a measuring tube having an inlet-side first end and an outlet-side second end which can be inserted into a pipe in a pressure-tight manner and comprises:
a nonferromagnetic support tube as an outer covering of the measuring tube,
a tubular liner located in a lumen of the support tube and made of an insulating material for conducting a flowing fluid isolated from the support tube, and
a reinforcing body embedded in the liner for stabilizing the latter;
a magnetic-circuit arrangement disposed at the measuring tube for producing and guiding a magnetic field which induces an electric field in the flowing fluid; and
a first electrode and a second electrode for picking up a voltage from the electric field.
The liner serves to chemically isolate the support tube from the fluid. In the case of support tubes of high electric conductivity, particularly in the case of metallic support tubes, the liner also serves to provide electric isolation between the support tube and the fluid in order to prevent the electric field from being short-circuited via the support tube.
Thus, by a suitable design of the support tube, the strength of the measuring tube can be adapted to the mechanical stresses exerted in the respective application, while by the liner, the measuring tube can be adapted to meet the chemical, and particularly hygienic, requirements in force for the respective application.
The liner, which is formed of plastic, is commonly made with an open-pore reinforcing body completely embedded therein, particularly a metallic reinforcing body. This reinforcing body serves to stabilize the liner mechanically, particularly against pressure changes and thermally induced variations of volume. JP-Y 53-51 181, for example, shows a tubular reinforcing body whose wall is provided with holes for receiving the liner material. This reinforcing body is located in and is coaxial with a support tube, and is completely surrounded by insulating material.
To optimize the density of the magnetic field and thus improve the sensitivity of the flow sensor, the end faces of the coil cores are designed as pole pieces with as large an area as possible and a given curvature. By shaping this curvature in a suitable manner, the density of the magnetic field in the measurement volume can be selectively optimized. This also optimizes the three-dimensional shape of the electric field and, thus, the dependence of the voltage induced in the fluid on the flow velocity of the fluid.
The three-dimensional shape of the magnetic field in the fluid and, thus, the accuracy of the flow sensor, besides depending on the form of the two end faces, are also determined by the distance between the two end faces. The farther the two end faces are apart, the weaker the electric field and the higher the sensitivity of the measured voltage to disturbances, such as changes in flow behavior or temperature variations in the fluid.
Therefore, to improve the accuracy of the flow sensor, on the one hand, the end faces should be spaced a minimum distance apart and, on the other hand, their curvature should be adapted to the respective optimum curvature as accurately as possible. In commercially available flow sensors, therefore, the pole pieces are shaped essentially according to the outer contour of the tube and are so disposed on the measuring tube that their end faces rest directly on the liner; see, for example, U.S. Pat. No. 4,825,703.
U.S. Pat. No. 5,664,315 discloses a method of manufacturing a measuring tube of an electromagnetic flow sensor whose inner surface is provided with a liner. Prior to the introduction of the liner into the support tube, an expanded-metal lattice which mechanically stabilizes the liner is fitted as a prefabricated reinforcing body. The liner is introduced by filling a liquefied insulating material into the measuring tube and allowing it to solidify. After having solidified, the insulating material surrounds the reinforcing body and thus forms the liner. The liner is preferably formed using injection-molding or transfer-molding techniques.
It is also common practice to install a completely prefabricated liner in the support tube. JP-A 59-137 822, for example, shows a method in which the liner is formed by softening an external plastic film and an internal plastic film surrounding a tubular, porous reinforcing body of high-grade steel.
It has been found that, on the one hand, liners of the above kind have a very high mechanical long-term stability, even in temperature ranges of xe2x88x9240xc2x0 C. to 200xc2x0 C. with corresponding jumps in temperature, but that, on the other hand, the introduction of a separately produced reinforcing body into, and its fixing in, the support tube are very costly and complicated steps in the manufacturing process. The cost and complication increase with increasing requirements placed on the accuracy of fit of the reinforcing body in the support tube.
It has also been found that with the coil cores disposed on the liner, particularly at a great ratio of the width of the support tube to the width of the reinforcing body for the liner and at low flow velocities of the fluid, increased measurement errors may occur.
It is therefore an object of the invention to provide a method of producing a liner of an electromagnetic flow sensor with a reinforcing body embedded therein which reduces the cost and complexity of the manufacturing process.
Another object of the invention is to provide an electromagnetic flow sensor in which arbitrarily shaped coil cores, particularly coil cores with curved end faces, each have one end positively fitted in the reinforcing body of the liner with a predeterminable depth.
To attain the first-mentioned object, the invention provides a method of manufacturing a measuring tube for an electromagnetic flow sensor, said measuring tube having an inlet-side first open end and an outlet-side second open end and comprising:
a nonferromagnetic support tube;
a tubular liner located in a lumen of the support tube and made of an insulating material; and
an open-pore reinforcing body embedded in the liner, said method comprising the steps of:
prefabricating the support tube;
forming a first sintering space in the lumen of the support tube by
inserting a first sintering mandrel with a smallest diameter greater than the smallest inside diameter of the liner into the lumen of the support tube and temporarily fixing it therein, and
closing the support tube in a sinter-tight manner, leaving at least one first filling aperture for a granular first material to be sintered;
forming the reinforcing body directly in the lumen of the support tube in such a manner that it fits said lumen, by
introducing the first material to be sintered into the first sintering space,
sintering the first material in the sintering space, and removing the first sintering mandrel;
forming a casting space in the lumen of the support tube by
temporarily fixing a casting mandrel having a smallest diameter not exceeding the smallest diameter of the liner in the lumen of the support tube, and
closing the support tube in a cast-tight manner, leaving at least one casting aperture for a liquefied insulating material; and
forming the liner directly in the lumen of the support tube by
introducing the liquefied insulating material into the casting space,
allowing the liquefied insulating material to penetrate into the reinforcing body, and
allowing the liquefied insulating material to solidify in the lumen of the support tube.
Furthermore, the invention provides an electromagnetic flow sensor comprising.
a measuring tube which can be inserted into a pipe in a pressure-tight manner and has an inlet-side first end and an outlet-side second end, and which contains
a nonferromagnetic support tube as an outer covering of the measuring tube,
a tubular liner of insulating material, located in a lumen of the support tube, for conducting a flowing fluid isolated from the support tube, and
an open-pore reinforcing body embedded in the liner for stabilizing the liner;
a magnetic-circuit arrangement disposed on the measuring tube for producing and guiding a magnetic field which induces an electric field in the flowing fluid, said magnetic-circuit arrangement comprising
a first coil,
a second coil,
a ferromagnetic first coil core magnetically coupled to the coils and having a first end face curved at least in sections, and
a ferromagnetic second coil core magnetically coupled to the coils and having a second end face curved at least in sections; and
a first electrode and a second electrode for picking up a voltage from the electric field,
the first coil core having a first end section inserted through a first wall opening of the support tube and fitted in a first coil-core seat of the reinforcing body in such a manner that the first end face is in positive contact with a first surface of the coil-core seat, and
the second coil core having a second end section inserted through a second wall opening of the support tube and fitted in a second coil-core seat of the reinforcing body in such a manner that the second end face is in positive contact with a second surface of the coil-core seat.
A first embodiment of the method of the invention comprises the steps of:
inserting, after the sintering of the reinforcing body and prior to the insertion of the first casting mandrel, a second sintering mandrel having a smallest diameter greater than the smallest inside diameter of the liner into the lumen of the support tube and temporarily fixing it therein in such a way as to form a second sintering space;
closing the support tube in a sinter-tight manner, leaving at least one filling aperture for a granular, second material to be sintered;
introducing the second material to be sintered into the second sintering space;
sintering the second material and thus enlarging the reinforcing body in the lumen of the support tube at least in sections; and
replacing the second sintering mandrel by the first casting mandrel.
A second embodiment of the method of the invention comprises the steps of:
forming, prior to the insertion of the first sintering mandrel,
a first expanded portion in the inlet-side first end of the support tube and
a second expanded portion in the outlet-side secondend of the support tube; and
before introducing in the first material to be sintered, closing the support tube in such a manner that after the sintering, the reinforcing body fills the first and second expanded portions at least in part.
In a third embodiment of the method of the invention, the first and second expanded portions are tapered toward the inside.
A fourth embodiment of the method of the invention comprises the steps of:
providing the support tube, prior to the insertion of the first sintering mandrel,
with a first wall opening for the insertion of a ferromagnetic first coil core having a first end face, and
with a second wall opening for the insertion of a ferromagnetic second coil core having a second end face; and
prior to the introduction of the first material to be sintered into the first sintering space, closing the first wall opening and the second wall opening temporarily in a sinter-tight manner with a first sintering closure and a second sintering closure, respectively.
In a fifth embodiment of the method of the invention, the sintering closures used to close the first and second wall openings in a sinter-tight manner are shaped so that after the sintering, the reinforcing body fills both wall openings at least in part.
In a sixth embodiment of the method of the invention, the sintering closures used to close the first and second wall openings in a sinter-tight manner are shaped so that after the sintering,
a first coil-core seat, starting from the first wall opening, is formed in the reinforcing body for the insertion of a first coil-core end section, starting from the first end face of the first coil core, and
a second coil-core seat, starting from the second wall opening, is formed in the reinforcing body for the insertion of a second coil-core end section, starting from the second end section of the second coil core.
In a seventh embodiment of the method of the invention, the sintering closures used to close the first and second wall openings in a sinter-tight manner each have a respective one of the coil cores temporarily inserted therein,
the first coil core being shaped and inserted in the first sintering closure in such a way that after the sintering, the first coil core fits the first coil-core seat, and that the first coil-core seat is in positive contact with at least part of the first coil-core end section, and
the second coil core being shaped and inserted in the second sintering closure in such a way that after the sintering, the second coil core fits the second coil-core seat, and that the second coil-core seat is in positive contact with at least part of the second coil-core end section.
In an eighth embodiment of the method of the invention, coil cores with end faces curved at least in sections are used.
A ninth embodiment of the method of the invention uses coil cores with end sections designed as a first pole piece and a second pole piece, respectively.
In a tenth embodiment of the method of the invention, coil cores with sintered end sections are used.
In an eleventh embodiment of the method of the invention, before the liquefied insulating material is introduced into the casting space, the first wall opening and the second wall opening are closed temporarily in a cast-tight manner with a first cap and a second cap, respectively, such that the insulating material fills both wall openings at least in part.
In a twelfth embodiment of the method of the invention, the caps used to close the first and second wall openings in a cast-tight manner are shaped so that after the solidification of the insulating material, the first and second coil-core seats for receiving the coil cores are formed in the liner.
In a thirteenth embodiment of the method of the invention, the caps used to close the first and second wall openings in a cast-tight manner each have a respective one of the coil cores temporarily inserted therein,
with the first coil core being shaped and inserted in the first cap in such a way that after the setting of the insulating material, the first coil core fits the first coil-core seat, and that the insulating material is in positive contact with at least part of the first coil-core end section, and
the second coil core being shaped and inserted in the second cap in such a way that after the setting of the insulating material, the second coil core fits the second coil-core seat, and that the insulating material is in positive contact with at least part of the second coil-core end section.
A fourteenth embodiment of the method of the invention comprises the steps of:
providing the first coil core with a cylindrical first coil and the second coil core with a cylindrical second coil prior to introducing the liquefied insulating material; and
using caps for closing the first and second wall openings in a cast-tight manner each having a respective one of the coil cores with a respective one of the coil cores temporarily fitted therein,
the first coil core with the first coil being shaped and fitted in the first cap in such a way that after the solidification of the insulating material, the first coil is embedded in the insulating material, and
the second coil core with the second coil being shaped and fitted in the second cap in such a way that after the solidification of the insulating material, the second coil is embedded in the insulating material.
A fifteenth embodiment of the method of the invention comprises the steps of:
providing the support tube with a third wall opening for the insertion of a first electrode and with a fourth wall opening for the insertion of a second electrode prior to inserting the first sintering mandrel; and
after the insertion of the first sintering mandrel, closing the third wall opening and the fourth wallopening temporarily in a sinter-tight manner with a third sintering closure and a fourth sintering closure, respectively.
In a sixteenth embodiment of the method of the invention, the sintering closures used to close the third and fourth wall openings in a sinter-tight manner are so shaped and dimensioned that during the sintering, both sintering closures extend into the lumen of the support tube.
In a seventeenth embodiment of the method of the invention, the sintering closures used to close the third and fourth wall openings in a sinter-tight manner are so shaped and dimensioned that during the sintering, each of the two sintering closures extends up to the first sintering mandrel.
In an eighteenth embodiment of the invention, before the liquefied insulating material is introduced, the third wall opening and the fourth wall opening are closed temporarily in a cast-tight manner with a third cap and a fourth cap, respectively, such that after having solidified, the insulating material fills the two wall openings at least in part.
In a ninteenth embodiment of the method of the invention, the caps used to close the third and fourth wall openings in a cast-tight manner each have a respective one of the electrodes temporarily fitted therein,
with the first electrode being shaped and fitted in the third cap in such a way that after the setting of the insulating material, the first electrode is fitted in the liner, and that the insulating material is in positive contact with sections of the first electrode, and
the second electrode being shaped and fitted in the fourth cap in such a way that after the setting of the insulating material, the second electrode is fitted in the liner, and that the insulating material is in positive contact with sections of the second electrode.
In a twentieth embodiment of the method of the invention, a support tube of high-grade steel is used.
In a twenty-first embodiment of the method of the invention, porous bronze is used as the first material to be sintered.
In a twenty-second embodiment of the method of the invention, polyfluorocarbon is used as the insulating material.
In a twenty-third embodiment of the method of the invention, the insulating material is introduced and allowed to solidify using a transfer-molding, compression-molding, or injection-molding technique.
In a first embodiment of the flow sensor of the invention, the first coil-core seat and the second coil-core seat are in positive contact with at least sections of the first coil-core end section and the second coil-core end section, respectively.
In a second embodiment of the flow sensor of the invention, the first coil-core end section and the second coil-core end section are designed in the manner of pole pieces.
In a third embodiment of the flow sensor of the invention, the reinforcing body is a sintered part.
In a fourth embodiment of the flow sensor of the invention, the liner is a molding or an injection-molded part which is in positive contact with at least sections of the first and second coil cores.
In a fifth embodiment of the flow sensor of the invention, the first coil and the second coil are wound on the first coil core and the second coil core, respectively, and are at least partly embedded in the insulating material of the liner.
One basic idea of the invention is to produce the liner directly in the support tube, i.e., in situ, rather than inserting it into the support tube as a prefabricated component.
Another basic idea of the invention is, on the one hand, to design the end faces of the coil cores arbitrarily within wide limits, particularly as pole pieces, and thus optimize the magnetic field in the fluid, and, on the other hand, to provide the reinforcing body with coil-core seats whose respective shapes correspond with the shapes of the end faces.
One advantage of the invention is that the reinforcing body can be fitted tightly into virtually any arbitrarily shaped lumen of the support tube in a simple manner. Through the additional formation of end-side expanded portions in the support tube and the filling of these portions with material for the reinforcing body, the reinforcing body, and thus the liner, can be centered and fixed in the support tube in a simple manner.
Another advantage of the method is that the liner with the embedded reinforcing body is produced already in its final form and position, so that both can be given virtually any three-dimensional shape required, particularly also a shape surrounding other components. Therefore, accurately shaped seats for the coil cores and feedthrough holes for the electrodes can be formed in the liner already during the sintering of the reinforcing body and during the introduction and solidification of the insulating material, with the inner surfaces of the through holes being covered by the insulating material of the liner if necessary. If the coil cores, or the cores with coils wound thereon, are disposed on the support tube already before the insulating material is introduced, they, too, can be embedded, wholly or in part, in the insulating material during the formation of the liner.
By embedding ferromagnetic materials in the reinforcing body in those areas where the magnetic field is to be coupled into the interior of the measuring tube during operation of the flow sensor, the pole pieces are directly integrable into the liner. If two or more ferromagnetic materials of different permeabilities are used, the three-dimensional shape of the magnetic field produced during operation of the flow sensor can be influenced and thus optimized.
A further advantage of the method of the invention is that the liner with the reinforcing body can be designed for arbitrary nominal diameters in any thickness and length, and thus with any mechanical strength and dimensional stability required, practically without additional technical complexity. This is possible since the reinforcing body, if a single sintering operation does not suffice to achieve the required mechanical strength, can also be sintered repeatedly. The reinforcing body can also be composed of two or more sintered layers formed successively in situ.
The invention will now be explained in more detail with reference to the accompanying drawings, which show embodiments of the invention. Like parts are designated by like reference characters. If necessary for clarity, however, reference characters have been omitted in subsequent figures. In the drawings: